The present invention relates to an apparatus for stacking and interconnecting integrated circuit die and/or multiple die segments of silicon, interconnecting the die and or multiple die segments on the edges of the stack using an electrically conductive polymer or epoxy, and mounting the stack of die on a BGA substrate.
For many years, electrical components such as transistors and integrated circuits have been made using wafers of semiconductor material, including silicon and/or germanium. Integrated circuits have been provided on the wafer using various techniques known as etching, doping, and layering. Individual integrated circuits that are provided on the wafer are referred to as die, and include contact points called bond pads for external electrical connections. Typically, the die on the wafer are separated from one another by cutting the wafer along boundaries defining the die. Once the die are cut from the wafer, they are referred to as chips or die, and are packaged for use. In recent years, the proliferation of more powerful electronic systems has led to an increased need for higher performance and higher density integrated circuit packages.
One method for creating higher density packages attempts to create an entire computer system on a single wafer using wafer scale integration (WSI) techniques. WSI technology attempts to laterally wire together all the die on a wafer using wires to interconnect the die. However, in order to create the necessary interconnections between the die, many wires are required that are extremely thin and difficult to create. Furthermore, the resulting interconnected die occupy a very large area, or footprint, on the electronic systems circuit board onto which the wafer scale integration device is attached for connection into the electronic system.
A second method for creating higher density packages attempts to reduce the area required for placing the chips on a circuit board by physically stacking the chips vertically. One chip stacking technique mounts individual die on ceramic carriers, encapsulates both the die and the carrier, stacks the carriers, and then mounts the stack on a printed circuit board. In this technique, all the die in the stack are interconnected by connecting the leads of the die to the printed circuit board via metal pins. This method results in an unusually high pin count on the circuit board which reduces the reliability of the circuitry because the high pin count increases the possibility that one of the many pins may become disconnected from the board.
Another chip stacking method uses a more complex process to stack die, as disclosed in U.S. Pat. No. 5,104,820 issued Apr. 14, 1992. This method modifies individual chips so that they may be stacked by adding a pattern of metalization, called rerouting leads, to the surface of the wafer. The rerouting leads extend from the bond pads on the chip to newly formed bond pads, and are arranged so that all the rerouting leads terminate on one side of the modified chip. Each modified chip is then cut from the wafer, and assembled into a stack. The stack is assembled in a manner such that all the leads of the modified chips are aligned along the same side of the stack. The side of the stack having the leads is then etched and polished so that a cross section of the leads on each of the modified chips is accessible. After the leads are exposed, a layer of metalization is applied to the leads along the side of the stack in order to electrically connect each of the modified chips in the stack. The stack is then mounted and connected to a substrate which in turn is connected to conventional circuitry.
This method of rerouting leads offers improvement in circuit density over prior methods but is complex and expensive. In addition, the rerouting leads extend over adjacent die, which are destroyed when the modified chip is cut out of the wafer. In this method, multiple die are sacrificed for every chip that is modified.
Another method for creating higher density circuits creates stacks from entire wafers, rather than individual chips, to form a wafer array. In some devices, the wafers in the stack are electrically interconnected using solid vertical columns of metallic conductive feed-throughs, such as copper. The use of solid feed-throughs to interconnect wafers may cause damage to the array due to differential thermal coefficients of expansion during thermal cycles. Furthermore, the process is costly and makes the wafers difficult to separate for repairs.
Other methods also exist to interconnect stacks of wafers, as disclosed in, for example, U.S. Pat. No. 4,897,708 issued Jun. 30, 1990, and U.S. Pat. No. 4,954,875 issued Sep. 4, 1990. These methods provide each wafer in the stack with coned-shaped through-holes which expose bonding pads on the wafers. The bond pads of the wafers in the stack are then electrically connected by either filling the through holes with electrically conductive liquid, or inserting an electrically conductive compliant material into the through holes, to provide a continuous vertical electrical connection between the wafers. While avoiding the disadvantages of using solid vertical columns of metal to interconnect wafers, the use of electrically conductive liquids and conductive materials requires special tooling to fill the through holes. Furthermore, for some applications, it may not be desirable to use stacks of entire wafers due to size constraints of the electrical device.
Individual semiconductor die are typically assembled in packages that allow the integrated circuit die to be attached to printed circuit boards and to allow electrical connections to be made between the integrated circuit die. There are many types of packages that are used for this purpose. The BGA package and the TSOP package are 2 types of packages in common use for assembling memory die and mounting the assembled die on a printed circuit board. There are a number of methods for stacking packaged integrated circuits, but in general, they suffer from a size disadvantage, and a performance disadvantage, due to the added electrical parasitics arising from the necessary lengths and characteristics of the inter-package interconnections. Due to the large physical size of the packages, there is a limit to the number of packages which may be stacked on top of each other, typically 2, to avoid thermo-mechanical problems. Stacks of packaged integrated circuits have been recently popular but take up too much board space, are too thick, and will not operate at the high speeds being required by advanced memory devices such as DDR2 and DDR3 DRAM.